Epoxy resins have long been used in various industrial applications, many of which must meet demanding performance criteria. For example, epoxy compositions are often used in the formation of composite materials and laminates, such as in the production of electrical laminates used in printed circuit boards (printed wiring boards, PWB). A requirement of this application, and many other applications, is highly flame resistance materials are required, and often a fire retardancy level of V-0 in the standard Underwriters Laboratory test method UL 94 is mandated.
Many types of flame retardant substances are known and many have been used in such applications, but the most commonly used materials to date have been halogen containing compounds, such as tetrabromobisphenol A. Typically, in order to reach the desired fire retardancy level (V-0 in the standard “Underwriters Laboratory” test method UL 94), levels of such bromine-containing flame retardant substances are required that provide a bromine content from 10 weight percent to 25 weight percent based on the total weight in the product.
There is an increasing interest in non-halogen containing flame-retardants that can not only provide the necessary flame retardancy, but which do so without having a negative impact on processing or physical characteristics such as mechanical properties, toughness, solvent and moisture resistance, etc. In certain applications the cure rate of an epoxy resin is an important consideration, e.g., a short cure time has obvious advantages in any manufacturing process but in some instances an epoxy composition may cure too quickly to allow for robust or exacting processing conditions. A flame retardant material that does not negatively impact on any of these important criteria is desirable; a flame retardant material that can enhance a desirable characteristic is highly desirable.
Phosphorus based flame retardants have been used as alternatives to halogenated flame retardants. Alkyl and aryl substituted phosphonic acid esters are compatible with epoxy resins but they are known plasticizers that can lower glass transition temperatures of some cured epoxy composition to unsatisfactory levels. They can also cause the resulting cured epoxy resin to absorb moisture, which can be very detrimental in many applications such as electrical laminates. Other phosphorus based flame retardant materials are known, for example, EP 0 754 728 discloses a cyclic phosphonate, EP 1 116 774 discloses the use of a hydrogen phosphinate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, in conjunction with triphenylphosphine oxide, but their use in some epoxy applications is often limited by process or performance difficulties or by increasing the costs of manufacture.
Phosphine oxide flame retardants have been used in epoxy resins. Phosphine oxide compounds containing reactive groups are known which are capable of being bound to a polymer, either into the backbone or attached to a pendant chain. For example, U.S. Pat. Nos. 5,817,736; 5,759,690; 5,756,638, 5,648,171; 5,587,243; 5,576,357; 5,458,978; 5,376,453; and 5,036,135 disclose the incorporation of a phosphorus based flame retardant compound into an epoxy resin, which is then cured with an amino cross-linker such as dicyandiamide, sulfanilamide, and the like.
U.S. Pat. Nos. 6,733,698; 6,740,732; 8,404,161; and 8,865,862 disclose hydroxyarylphosphine oxide flame retardants which can react with epoxy groups and serve as cross linkers for epoxy resins. Alternately, these compounds can be derivatized prior to incorporation into epoxy resins, for example, reaction between the phenoxy group and epichlorohydrin provides a useful epoxy functionalized material.
U.S. Pat. No. 6,403,220 and U.S. Pat. No. 6,645,631 disclose a general process for the preparation of electrical laminates useful in printed circuit boards, which process comprises (1) applying or impregnating an epoxy-containing formulation onto or into a substrate, such as a woven or nonwoven fiber mat containing, e.g., glass fibers or paper, which substrate is then (2) heated at a temperature sufficient to draw off solvent from the epoxy formulation and optionally to partially cure the epoxy formulation. This heating step is known as “B-staging” and the product is known as a “prepreg”, which, as a result of “B-staging”, is more easily handled in the subsequent manufacturing steps wherein (3) one or more sheets of prepreg are stacked or laid up in alternating layers with one or more sheets of a conductive material such as copper foil, if an electrical laminate is desired, and pressed at elevated temperature and pressure for a time sufficient to cure the resin and form a laminate. Common temperatures for the “B-staging” step are from about 90° C. to about 210° C. for a time ranging from about 1 minute to about 15 minutes, but other temperatures and times can be used.
The formation of laminates can be a very exacting process. One difficulty encountered in preparing laminates such as those found in circuit boards, is that some curable epoxy compositions have gel times, i.e., the time period during which a prepreg remains partially cured and easily handled, of less than 180 seconds, sometimes much less, as observed by the stroke cure test, which complicates production of prepregs and the final laminates. Gel times of greater than 250 seconds, for example 300 seconds or greater are much more desirable. Some epoxy resin formulations cure at an unacceptably fast rate to allow for proper handling. Copending U.S. patent application Ser. No. 13/455,414 discloses that certain phosphorus containing salts that are also known as flame retardants, e.g., salts of phosphinic acids, can retard the cure rate of epoxy compositions when used at very low levels, e.g., less than 2 wt %. Of course one does not want overly extend the curing time for commercial and quality reasons.
Other flame retardants derived from phosphorus containing acids and salts are known. For example, copending U.S. patent application Ser. Nos. 14/337,500 and 14/592,472, disclose that products obtained by heating certain alkylphosphonic acid metal salts, such as aluminum salts, calcium salts, zinc salts etc., at temperatures in excess of 200° C. are thermally stable at temperatures above 400° C. and are readily incorporated onto polymer compositions to provide excellent flame retardant properties. It has been found that these highly stable and compatible flame retardants can be added to many epoxy compositions at levels up to 50% and often higher, e.g., 55% or 60% by weight in a straightforward manner without interfering with processing, without volatizing or exuding from the cured or precured composition, without negatively impacting desired physical properties, while having a negligible effect on cure rates. Higher concentrations of the flame retardant in certain resins are possible depending on the resin formulation, the processing or application methods and the physical properties of the pre-cured and cured resin. These compositions are typically halogen free and are well suited for use in preparing, highly flame retardant laminates, composites and the like, including e.g., circuit boards and prepregs for circuit boards.